Logical reasoning skills are important for a successful mathematical learning and in students' future career. These skills are essential for a primary school teacher, because they need to explain solving methods and solutions to their pupils. In this research we studied pre-service primary school teachers' logical reasoning skills. The results show that only one third of the students gave an argumentation for their answer: a very small percentage gave a complete, correct argumentation for the solution of the problem; one fifth of the students had an incomplete argumentation, missing important steps of the logical reasoning; and one tenth of the students made mistakes in their reasoning. These results highlight the importance of developing future primary school teachers' logical reasoning skills.